Ink-saturated endless ribbons are widely used as unicolor or multicolor ink ribbons for the wire dot printers of computers, word processors or like devices. Such endless ribbons are manufactured by producing a plain-woven fabric using a nylon or polyester multi-filament yarn as warp and as weft, slitting said plain-woven fabric to a predetermined width in such a manner that the direction of warp coincides with the longitudinal direction of the ribbon, and joining both ends of the ribbon with each other to make it endless.
There also is known the so-called seamless ribbon which is manufactured by producing a tubular, jointless (namely seamless) plain-woven fabric by double or tubular weaving and then fuse-cutting the same in the radial direction. In this case, contrary to the above-mentioned case of obtaining endless ribbons from a plain-woven fabric, the direction of weft of the fabric becomes the longitudinal direction (circumferential direction) of the ribbon. Seamless ribbons are advantageous just because, unlike endless ribbons, they have no junction.
In manufacturing such seamless ribbons, nylon or polyester multifilament yarn is generally used as warp and weft.
When a 40d/34f multifilament yarn, for instance, is used for weaving, the mean yarn density is typically selected as shown below in Table 1. The most representative is a quasi-high density construction with a warp density of about 146 yarns/inch, a weft density of about 148 yarns/inch and a total density of about 294 yarns/inch.
TABLE 1 Warp density A Weft density B A + B (yarns/inch) (yarns/inch) (yarns/inch) Ordinary density 132 142 274 product Quasi-high density 146 149 294 product High density 190 150 330 product Very high density 180 162 342 product
As regards the ribbon width, unicolor ribbons are relatively narrowand, as the number of colors increases, the width is increased accordingly. The circumferential length of the ribbon may be selected arbitrarily, although the upper limit to the ink ribbon circumferential length is by itself defined when the ribbon is intended to be loaded into a standardized cassette. Though dependent on cassette size, the ribbon is loaded in two different ways: a ribbon with a long circumferential length (e.g. 2600.about.3000 mm) is folded in a zig-zag fashion and encased in that condition, while a ribbon with a short circumferential length (e.g. 200 mm or 300 mm) is not fold but simply mounted on the roll of a compact cassette.
When the ink ribbon contained in a cassette and making a round arrives at the head, it is struck by the printer needle and the ink in the remaining part of the ribbon diffuses to resume homogeneity.
In impact printing, a platen, a printing paper, a perforated mask, an ink ribbon and a head are arranged in that order, a dot printer needle protruding from the head is thrust against the ink ribbon to thereby strike the ink ribbon, through holes of the mask, against the paper, with the resulting shock being absorbed by the platen.
As the ink ribbon, a ribbon manufactured by laminating a woven fabric with a polymer film or another woven fabric and fusion-cutting the laminate to the width of a ribbon are known. For example, JP Kokai S60-19578 describes a ribbon for typewriter or other use which has been fabricated by integrating both lateral edges of two ribbons superposed one on the other.
Japanese Patent Publication H-01-26349 (JP Kokai S57-93187) discloses a method of manufacturing ink ribbon base cloths which comprises reducing, by 2 to 10%, the thickness, after scouring and setting, of a high density woven fabric composed of 20 to 120 denier synthetic fiber multifilament yarns as warp and weft and having a warp density of 150 to 220 yarns/inch and a weft density of 100 to 140 yarns/inch by pressing under heating, without rendering the same film-like, to render the same apparently reed mark-free. The thickness reduction is effected by hot pressing using a heating and pressing means, at least two rollers of which are driven at different speeds in a ratio of 1:1.1 to 1:2.0.
JP Kokai H11-43840, the Japanese Patent application filed by the present applicant on which U.S. Pat. No. 5,984,547 is based, discloses a seamless ink ribbon comprising a seamless woven fabric constructed by using multifilament yarns for both warp and weft and reduced in thickness by 15.about.50% from the original thickness by static hot pressing without shearing force application.
JP Kokai H11-78186, recently laid open, discloses an ink ribbon comprising a base or substrate ribbon made of two superposed webs having dissimilar characteristics, with the web of greater abrasion resistance being disposed on the printing paper side and the web of greater impact transmissivity being disposed on the printer head side. To implement this base ribbon construction, the following arrangements are mentioned.
The thickness of the substrate web on the printing paper side is designed to be larger and that of the web on the printer head side smaller. PA1 The fiber weight or density per unit area of the substrate web on the paper side is designed to be higher and that of the substrate web on the printer head side lower. PA1 The rate of crimp of the yarn for the substrate web on the printing paper side is designed to be lower by 2.about.5% as compared with the web on the printer head side.
Paragraphs [0024], [0036], [0048] and [0078] and FIG. 4 of the above-mentioned official gazette JP Kokai H11-78186 describe and illustrate the manner in which a ribbon made of two woven substrate webs superposed one on the other is pressed into a flat substrate by means of heat and pressure. In the description of the prior art in Paragraph [0057] et seq. of the same gazette, there is a description of the conventional single-layer ink ribbon.
In the case of an ink ribbon with a short circumferential length which is loaded in a compact cassette, the frequency of the ribbon being subjected to the impact of the printer needle is considerably high because of its short circumferential length, with the result that the ribbon is damaged early and must be exchanged at short intervals. Thus, although the ink ribbon still holds a sufficient amount of ink, it must be discarded as the number of characters which can be printed with one ink ribbon (the number of sheets of paper which can be printed) is considerably decreased.
The above shortcomings can be overcome to some extent by using an ink ribbon made of a fabric/polymer film laminate or a fabric/fabric laminate, fuse-cutting it to the width of a ribbon and splicing the resulting strip but such a measure is far from providing a fundamental solution to the problem.
The invention disclosed in Japanese Patent Publication H1-26349 referred to above relates to an endless ribbon constructed by splicing both ends of a ribbon and is intended to liquidate the reed mark of a high-density woven fabric. In addition, this invention is concerned with a single-layer ribbon which has been reduced in thickness by 2.about.10% from the original thickness by means of two rollers varied in peripheral speed and it is stated that if the reduction in thickness exceeds 10%, the profile of the ribbon is flattened to approach to that of a film and the ink absorbability is adversely affected.
JP Kokai H11-43840 referred to above, a patent application field by the present inventors, describes only a single-layer seamless ink ribbon. Here, subsequent investigations made by the present inventors revealed that there is a limit to increasing the printing life even if the thickness is reduced by static hot pressing, thus leaving room for further improvement.
Furthermore, in JP kokai H11-291593, which had not been disclosed yet as of the filing date of the Japanese application of this invention, the present applicant disclosed a multi-layer ink ribbon manufactured by hot-pressing a substrate laminate consisting of two webs each woven using multifilament yarns for both weft and warp, said substrate laminate having been compressed and set by 25.about.65% thinner than the sum of thicknesses of the respective webs prior to hot pressing and the laminate has been fuse-cut to the ribbon width. However, it has been discovered that when two webs are hot-pressed in superimposition as it is the case with this multi-layer ink ribbon, the fiber of the substrate fabric is partially fused and deformed by the heat and pressure to cause non-uniformity of the interfilament voids necessary for holding the ink with the inevitable local variation in the amount of ink saturation, so that uneven prints tend to take place in the course of printing.
JP Kokai H11-78186 referred to above describes a mode in which a ribbon made of superposed webs is flattened by the application of heat and pressure (Examples 1 and 2; FIG. 4) and a mode in which the conventional single-layer ribbon is flattened by heat and pressure (prior art) but the technology disclosed has disadvantages similar to those of the technology described in JP Kokai H11-43840 (U.S. Pat. No. 5,984,547) and the technology described in JP Kokai H11-291593 (not disclosed yet as of the filing date of this application) both filed by the present applicant.